The invention relates more particularly to the frequency modulated luminance signal used in the VHS or 8 mm video tape recorders prior to the recording of this signal on the magnetic tape.
For example, with VHS models, the frequency modulation of the luminance signal has, as a minimum frequency, a frequency of 3.8 MHz which corresponds to the blanking level of the synchronization pulse and as a maximum frequency, a frequency of 4.8 MHz which corresponds to the maximum white level. Those two frequencies f.sub.min and f.sub.max have a tolerance of 100 kHz as well as for their difference which also has a tolerance of 100 kHz, those tolerances being assigned by the PAL and SECAM standards for VHS.
It is of common use to carry out a setting of those frequencies at the factory, before putting the sets for sale. The adjustment is made once for all, manually, by means of two potentiometers. The first potentiometer is connected with the gain setting input of an amplifier which receives the luminance signal, and a second potentiometer is connected with the input of an adder which receives, on another input, the output signal of this variable gain amplifier, the output signal of this adder being applied to the input of a voltage-controlled oscillator which permits the frequency modulation of the luminance signal. The setting of the frequency f.sub.max is made from a video signal corresponding to the white level, this signal being injected into an input of the amplifier which receives on another input a voltage level corresponding to the blanking level which determines the frequency f.sub.min of demodulation. The setting by means of the first potentiometer permits to increase or decrease the gain on the difference between the input levels. The setting of frequency fmin is carried out by means of the second potentiometer which resets the voltage level received by the adder at one input in order to obtain the desired f.sub.min frequency.
The settings carried out by this method are tedious and lack accuracy. Further, they are carried out only once for all from a specific video signal.